Hi Rebecca,
I have stuggled with this problem too. For many years. I have found a few techniques that help me.
1) I imagine that the negative thoughts are boats sailing down a river. Now normally as one of these "boats" sails by I start to worry about it and I "get on board" and find myself "sailing down that river." In this exercise I practice watching the thoughts come down the river and watching them sail by without getting on board. So instead of starting to worry about the negative thought. I visualize it as a boat and just let it sail by without getting on. And I just wait for the next boat to sail by. It's kind of a meditation/visualization I guess. It helps me though.
2) I develop counter thoughts. So when the negative thought comes, ie: "you'll never get that job..." I now have a counter thought that counteracts the negative thought. for example "that's not something I can worry about right now. Everything will work out regardless of what happens."
I hope this helps. Thanks for sharing about this. This has really been an issue for me too lately